The idea is to turn the lights off bar one single light, or one single candle for one hour from 10pm to 11pm - it's an act of rememberance and commemoration. The foreign secretary at the time of war breaking out said, 'the lamps are going out all over Europe; we shall not see them lit again in our life-time.' - this quote is the inspiration for the RBL 'Lights out' event. The time was chosen to co-incide with the time war was declared in 1914.
I see it as a focus for personal reflection - a time when the whole country has one thought (like 11am on Armistice Day), and I'll be taking part.